This invention relates to an optical inspection system and particularly to one for evaluating surfaces of large contoured panels through quantitative evaluation of a pattern reflected by the surfaces.
The inspection of sheet metal panels for cars has been approached in a variety of ways. The most commonly used method of inspecting panels for shape errors has been the so-called "highlighting" method. The highlighting method has been used in a subjective manner through the use of skilled inspectors who move around the panels to get just the right reflection off the panel that highlights subtle defects.
The inspection of car panels for shape defects has been a popular and active topic for a number of years. There have been numerous techniques applied to this inspection problem, ranging from radar like measurements to purely visual inspection methods. The problems associated with automating the inspection of body panels has two primary considerations which must be addressed: the quantification of the defects, and the inspection technology to be automated. The general nature of the defects fall into such categories as dents, scratches, waviness, wave edge, hi/lo seam, recoil, metal finish, streamers, and dirt pimples. The current inspections for these features is very subjective in nature.
The most common inspection for dents and lows uses an oil type of "highlighting" fluid to provide a glossy finish similar to that seen on a finished painted car. The glossy finish provided by the highlighting makes the panel appear mirror like, permitting a view of an object beyond the panel to be seem in the reflection off the panel. The inspector typically looks at the edge of colored florescent tubes placed around the panel as reflected from the surface of the panel. If the inspector detects the presence of a "wave" in the edge of the light as he or she moves their head past an area, it is marked as having a dent or "error" in the contour. This is the same type of effect of distortion often seen in older window glass. The surface of the panel is acting as a mirror. The light reflected from the panel is deviated according to the surface slope in accordance with the laws of reflection, creating a distorted image of the light fixture, just as if the inspector was looking at a reflection in a "fun house" mirror.
Actually, the image the inspector sees in the reflection off the highlighted panel is generally always distorted in accordance to the shape of the panel. Therefore, the inspector is looking for some "out of the ordinary" distortion, such as a sudden change in an area expected to be flat. In areas which have sudden contour changes, the inspector may actually be looking for a flat area where the surface is expected to be very curved. Markings, surface finish irregularities or an uneven highlighting fluid application can further complicate this inspection with factors which would be different for a painted surface. Just in this simple example, it is obvious that no one decision is necessarily used to inspect the panel. The final analysis of what features or distortions in the reflected image seen off the panel are important is left up to the inspector.
The car buyer sees a similar effect. Many people become aware of irregular contours on the hood of their car when driving under power lines, by the way the reflection of the lines moves over the hood. Seeing stray dips and deviations in the reflection gives a perceived impression of poor quality. A smooth, geometrically uniform reflection is seen as good quality. Different areas of the car are viewed at different angles, relative to the normal to the surface, making the detection of such defects more or less likely. The reason the angle of view becomes important is the same reason structured light gages are sensitive to angle.
If a straight line of light illuminates the surface of a part, at some angle, and is viewed from some different angle, the line will be seen as deviated from a straight line by the surface contour, into a shape which follows some cross-section through the surface. The sensitivity of the projected line to this effect changes as the sum of the tangent of the illumination and viewing angles, as measured from the average normal to the surface. If the line is viewed along the same path it is projected, no deviation is seen if the surface is diffuse. If a specular (mirror like) reflection is used, then the reflected light is deviated by twice the surface slope to the projected line. The resolving power for the average human eye is about 2 minutes of arc (about 0.6 milliradians) or 1 part in 1700. This means that a surface slope error of 2 minutes of arc on a specular surface would be just resolved by the average human observer. Human perception, however, is most sensitive to changes from the surroundings. Therefore, a dent or flat area seen in one area which has little shape to it may not be perceived in an area of sharp transitions.
Human perception is also an important consideration in scratches or small dirt pimples (small bumps which typically occur when dirt is present in the pressing operation). By the resolution criterion above, we know the average observer can resolve about 0.04 inches (1 millimeter) at about 65 inches (1625 millimeters), or by rule of thumb, a person can resolve about 0.02 inches or 0.5 millimeters at an arm's length distance. This does not mean smaller features cannot be seen. Just as we can see stars in the sky which are too distant to resolve (such as a double star), small features can be detected, but will appear not smaller than the 0.02 inches (0.5 millimeters) nor could we distinguish two scratches below our resolution. As the size of a scratch decreases, the amount of light it scatters decreases as its area. The limited resolution of the human eye further reduces the density of light at a point by spreading it over the resolution spot of the eye, thus reducing the contrast of the defect as seen. This defect detection capability is effectively a measure of the ability to collect visual feature information. Any inspection system should take these factors into account to better compare to what would be seen by the customer.
The quantification of the parameters which affect the perceived quality of the body panel is the first step which must be taken to provide a meaningful panel inspection system. Data based on thresholds that may change with dirt levels, evenness of the oil highlight, or light level fluctuations are subjective in nature and can not be easily calibrated. Therefore, an inspection made in one plant may vary greatly from an inspection done in another plant by such methods.
It would also be desirable to produce a quantitative record of the nature and severity of the defects which could be used for documentation purposes. The range and diversity of the defects creates an interpretation task which is a major undertaking in itself. The intent in this invention is not to address the major task of defect interpretation, but to provide a reliable, quantitative means of characterizing the defects seen by the standard highlighting methods.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.